Anyone a Master of Physics?

Originally posted by HolyT
I don't think you're talking about low pressure, since the pressure differences by themselves don't lift roofs.

I think he's talking about low pressure. The Bernouilli effect is a big clue (pressure decreases as speed increases). So in high winds the pressure above the roof is lower than the pressure below it.

Which explains the point about the windows as well. Opening the windows allows the pressures to equalise.

Before you clarify, I'll assume you're referring to the effects of the straight-line winds. Also, refer to this site regarding opening or closing windows in or near a tornado: http://www.windows.ucar.edu/tour/link=/earth/Atmosphere/tornado/myths.html&edu=elem
When wind flows over a roof of a house in 3 dimensions, the wind flowing over the house (and around it, for that matter), must "meet back up" with the wind that bypassed the house (at core, it's a conservation of mass issue). Bernoulli's principle (forgetting many details) tells us that higher-speed fluid has a lower pressure (static pressure) associated with it. Or, generally, the sum of static and dynamic pressure (1/2 * density * velocity squared) remains constant. Velocity goes up, static pressure goes down. The space above the roof of the house becomes a lower-pressure area (like the region above an airplane's wing) and the roof could possibly lift up (as the wing does).
For part B: I don't know how much it matters in reality, but if your house is pretty well sealed, and pretty close to equal pressure with the prevailing static pressure locally, and then the roof comes off, suddenly the inside of your house would be at a higher pressure than the region above you, and would equalize (air and roof sucked upward). The air outside your windows at a lower level would also be at a higher pressure, and might try to equalize pressure by forcing its way into the suddenly lower-pressure region inside your house, imploding your windows. But I imagine your house would have had to be pretty well sealed before the event, and your windows would have to be pretty weak. If your roof came off, you probably already had bigger structural problems to worry about!
I'm sure someone else could have explained this in a less wordy way!

Originally posted by mtthw
I think he's talking about low pressure. The Bernouilli effect is a big clue (pressure decreases as speed increases). So in high winds the pressure above the roof is lower than the pressure below it.

Which explains the point about the windows as well. Opening the windows allows the pressures to equalise.

I should have said that I didn't think he was talking about the prevailing lower atmospheric pressure as compared to average pressure.

See also: http://www.tornadoproject.com/myths/myths.htm

Originally posted by Ramned
Solids / Fluids

[b]9.Tornadoes and Hurricanes often lift the roofs of houses. (A) Why does this happen? Explain; use the Bernoulli Effect to explain why. (B) Why should you keep your windows open under these conditions?[/b]

poorly posed question.
tornadoes remove roofs because of presure differenes
hurricanes do so because of sustained force
the mechanics are completely different.

That was a confusing question - too broad...new question to follow!

it's easier than all that.

Air moving perpendicular to an object is not "pushing against" the object with the same force as air on the other side of the object that is not moving perpendicular to it.

Air moving over the house (straight line with the ground) is not "pushing down toward the ground" with as much force as air from the ground is "pushing upward."

Now, where is that air that is pushing up? It's INSIDE the house!

Originally posted by Ramned
[b]9. When you are in a small car on a freeway, and a big truck speeds past you, why do you feel like you're being pulled toward the truck?[/b]

Errr... pardon if I get this wrong...

Wind speeds? Bernoulli's Principle?

Air movement @ higher speeds = lower pressure...right?

High speed air b/t vehicles, caused by the semi, is at lower pressure than that on the other side of the vehicle...I think.

Hence, the pressure tries to equalize by pulling the smaller car towards it...

Is that right?

And I heard somewhere that this principle can be evidenced better if you were standing still on the side of the highway, like a car breakdown or something, rather than moving. Because the higher the difference in speed between the two of you, the faster the wind will blow by, creating a much lower pressure than if you both were in motion, which would lessen the blow of the wind current.

Errrr...

Thank you...

'Stein...

PS. Same thing works with ships correct? SInce it works in both fluids and gases.

Originally posted by Ramned
[b]9. When you are in a small car on a freeway, and a big truck speeds past you, why do you feel like you're being pulled toward the truck?[/b]

As the truck moves forward it pushes air out of the way. Air rushes in to fill the area of low pressure behind the truck. In so doing it can exert force on nearby vehicles.

Sorry, I was replying to question #4 and didn't notice there were pages ahead... lol
Begginer's mistake. Good thread though lol

That musta been too easy 😉 Everyone got it.

The next question to follow - #10. Once (IF) you get it correct, which I am sure one of you will, there will be an Intense problem that will be based on the 10 conceptuals - once that is figured, the next 10 come (harder set!)

Good luck...

Thermal Physics

10. In astronomy, the temperature at the core of a star is given to be 1.5 X 10^7 degrees. How would you respond if an astronomer studying this star asked you if the temperature is Celsius of Kelvins? Explain.

Originally posted by Ramned
Thermal Physics

[b]10. In astronomy, the temperature at the core of a star is given to be 1.5 X 10^7 degrees. How would you respond if an astronomer studying this star asked you if the temperature is Celsius of Kelvins? Explain.[/b]

I'd ask why he is arguing over a few hundred degrees when the temperature given is obviously only to the nearest ... 10^6 (?)

Originally posted by wolfgang59
I'd ask why he is arguing over a few hundred degrees when the temperature given is obviously only to the nearest ... 10^6 (?)

😵

Perfect answer! You walked right into it.

It doesn't matter because they differ by only ... 273? ... degrees. Insignificant for temps in ordder of 10^7 degrees.

Good stuff. The grand problem follows 😉